Aircraft and Carbon Dioxide Emissions

Climate change requires significant changes to our lifestyle, and one of the more tricky problems to solve is air travel. Interestingly, you will find many environmentalists always telling everyone to cycle, but then spend tens of thousands of air miles going to environmental conferences. So, what can we do?

One solution is to reduce air travel. And there is no need in principle to adopt Greta Thunberg’s solution of sailing over the Atlantic. With a bit of investment, high speed rail can get you between the centres of reasonably close cities faster than aircraft, when you include the time taken to get to and from airports, and time wasted at airports. We can also reduce travel, but only so far. At first sight, things like conferences can be held online, but there are two difficulties: time-zone differences encourage doing something else, and second, the major benefit from conferences is not listening to set talks, but rather meeting people outside the formal program. For business, facing each other is a far improved way of negotiating because the real signals are unspoken. 

Some airlines are trying to improve their environmental credentials by planting trees to compensate for the carbon dioxide they emit. That is very noble of them, but apart from the fact it is their money doing it (and often it is not – it is the passengers who feel conscious stricken to donate more money for planting) it is something that should be done anyway. 

There has been talk of building electric aircraft. My personal opinion is this is not the solution. The problem is in terms of unit weight, jet fuel contains at least thirty times the energy density of the best batteries available. Even worse, for jet fuel, as you go further, you get lighter, but not with batteries. You could make a large aircraft fly, say, 1,000 to 2,000 km, as long as you did not want to carry much in the way of passengers or cargo. With thirty times the fuel weight for a long distance flight your aircraft would never get off the ground. However, the Israeli firm Eviation has developed a small electric aircraft for a load of 9 persons (plus two crew) powered by 920 kWh batteries with operating costs estimated at $200/hr. The range is about 540 nautical miles, or about 1,000 km. That could work for small regional flights, and it will be available soon.

Another option to be offered by Airbus is the E-Fan-X project. They will take a BAe 146 craft, which usually carries about 100 passengers, and which usually is powered by four Honeywell turbofan engines, and replace one of the inner ones with an electric-driven 2 MW propulsion fan motor. The idea is the takeoff, where the most power is required will use the normal jets, but the electric motor can manage the cruise. 

An alternative is to reduce fuel consumption. One possibility is the so-called blended wing, which is being looked at by NASA. This works; an example is the B2 bomber, however while it reduces fuel consumption by 20% it is most unlikely to come into commercial use any time soon. One reason is that there is probably no commercial airport that could accommodate the radically different design. It would also have to have extensive examination because so far the design has only had military applications, in which only very specific loads are involved. In principle, this, and other designs can reduce kerosene usage, but only by so much. Maybe overall, 25% is achievable, which does not solve anything.

Uranium 235 has an energy density that leaves kerosene for cold, but which airport wants it, and would you board it anyway? It could presumably be made to work, but I can’t see it happening anytime soon because nobody will take the associated political risk.

That leaves hydrogen. 1 kg of liquid hydrogen can provide the same energy as 3 kg of kerosene, so weight is not the problem, but keeping it cold enough and maintaining pressure will add weight. It cannot be stored in the aircraft wings because of the volatility. To keep it cold it is desirable to have minimum surface area of the tank. However, it is reasonably clean burning, giving only water and some nitrogen oxides. For a Boeing 747-400 aircraft, the full fuel load is 90 tonne less, but because the tanks have to be in the fuselage, they occupy about 30% of the passenger space.

That may work for the future, but the only real way to power current aircraft is to burn hydrocarbon fuel. More on that next week.